In recent years, a problem has been raised about harm to the environment and the human body caused by particulates contained in exhaust gas discharged from internal combustion engines of vehicles such as automobiles, buses, and trucks, and constitution machines and the like. A variety of honeycomb filters have been proposed, in which exhaust gas is allowed to pass through porous ceramic, and particulates therein is then collected so that the exhaust gas is purified.
FIG. 1A is a perspective view that schematically shows one example of such a honeycomb filter, and FIG. 1B is a cross-sectional view of the honeycomb filter shown in FIG. 1A. Further, FIG. 2A is a perspective view that schematically shows one example of a porous ceramic member constituting the honeycomb filter shown in FIG. 1A, and FIG. 2B is a cross-sectional view taken along line A-A of FIG. 2A.
As shown in FIG. 1A, in a conventional honeycomb filter 90, a plurality of porous ceramic members 95 made of silicon carbide or the like are bound to one another through a sealing material layer 91 to constitute a ceramic block 93, and a sealing material layer 92 is formed on the outer periphery of the ceramic block 93.
Further, as shown in FIGS. 2A and 2B, in each of the porous ceramic members 95, a number of through holes 96 are placed in parallel with one another in the length direction so that the partition wall 98 that separates through holes 96 functions as a filter. Namely, as shown in FIG. 2B, each of the through holes 96 that are formed in the porous ceramic member 95 has one end thereof, on either an exhaust gas inlet side or outlet side, sealed with a plug 97, so that exhaust gas having flown into one through hole 96 necessarily passes through the partition wall 98 separating the through holes 96, and then flows out of another through hole 96.
In an exhaust gas purifying device, the honeycomb filter 90 with such a structure is installed in a casing connected to an exhaust passage in an internal combustion engine through a mat-like holding sealing material, and particulates in the exhaust gas discharged from the internal combustion engine are collected by the partition wall 98 as passing through this honeycomb filter 90, leading to purification of the exhaust gas.
However, in the exhaust gas purifying device with the above-mentioned structure, as shown in FIG. 1B, the honeycomb filter, installed in the casing through the mat-like holding sealing material, is constituted by forming the sealing material layer 92 on the outer peripheral face of the ceramic block 93. This structure might cause displacement of the honeycomb filter in the casing when pressure that is applied on the end face of the exhaust gas inlet side of the honeycomb filter is raised with an increase in collected amount of the particulates contained in the exhaust gas, or when the holding power of the honeycomb filter, generated by the mat-like holding sealing material, inside the casing is reduced due to expansion of the casing to a size larger than that of the honeycomb filter through heating to a high temperature.
Upon occurrence of displacement of the honeycomb filter inside the casing as described above, the length direction of the honeycomb filter might become non-parallel with the flowing direction of the exhaust gas to cause reduction in particulate collecting efficiency, or the honeycomb filter might be brought into contact with the casing to generate cracks in the honeycomb filter. Further, the mat-like holding sealing material might droop onto the end face of the exhaust gas inlet side of the honeycomb filter, closing the through holes exposed at this side to cause reduction in exhaust gas purifying efficiency.
To deal with this, JP-A 2003-262118 discloses a honeycomb structural body with the cross-sectional shape thereof changed from an exactly circular shape to a flat shape to adjust the circularity for improving the holding power of the honeycomb structural body. Further, JP-A 2001-329836 discloses a honeycomb structural body having irregularities formed on the outer periphery thereof to adjust the circularity. When each of these honeycomb structural bodies is installed in a casing through a mat-like holding sealing material as an exhaust gas purifying device, the mat-like holding sealing material fits into the recesses on the outer periphery of the honeycomb structural body so as to fill the recesses, thereby to improve the holding power of the honeycomb structural body inside the casing.
JP-A 2003-260322 discloses a honeycomb structural body, in which a bonding layer provided in the slanting direction of through holes is set thick so as to increase the isostatic strength.
The contents of JP-A 2003-262118, JP-A 2001-329836 and JP-A 2003-260322 are incorporated herein by reference in their entirety.